Just a heads-up;
I'm going to release a new draft of Link soon, and document my
preferred approach: grandfathering in "alternate stylesheet" as
meaningful when they're serialised together, but discouraging other
such special relationships (e.g., "up up").
I'll then take that to the HTML5 WG as an issue to see if we can come
to some agreement about it.
Cheers,
On 08/04/2009, at 9:10 AM, Mark Nottingham wrote:
> I've been discussing the link draft with Ian Hickson, who points out
> that in HTML4, there's a difference between
>
> <link rel="stylesheet" href="a"/>
> <link rel="stylesheet alternate" href="a"/>
>
> and
>
> <link rel="stylesheet alternate" href="a/>
>
> (see <http://www.w3.org/TR/html401/present/styles.html#specifying-external
> > for the background of why these are different)
>
> In the current link draft, there isn't any way to express the
> difference between these; the underlying model is
>
> [ context ] ---[ relation type ]---> [ target ]
>
> where 'relation type' is singular.
>
> To accommodate this use, the model would need to be something like
>
> [ context ] ---[ list of relation types ]---> [ target ]
>
> noting that there may be more than one list of relation types
> between any context and target.
>
> Personally, I think that it would be only in pathological cases that
> it would be necessary to know the difference between the two (i.e.,
> real world Web pages will not point to a URI as both a stylesheet
> and as an alternate for themselves, so it's safe to say that even
> the first example above means that "a" is an alternate stylesheet).
>
> However, it is important for Link to interoperate well with HTML4.
> Also, the HTML5 folks plan to use this model for other purposes
> (e.g., "up up" to indicate a parent of a parent).
>
> The practical impact of making this change is that serialisations of
> links won't be able to collapse multiple relation types between two
> URIs into one link; they'll have to be separate to allow this
> interpretation.
>
> So, for example, if you have link types ['w', 'x', 'y z'] between A
> and B, it will have to be serialised as
>
> Link: <B>; rel="w"
> Link: <B>; rel="x"
> Link: <B>; rel="y z"
>
> in HTTP headers, NOT
>
> Link: <B>; rel="w x y z"
>
> because that's ambiguous.
>
> The alternative is to say that the 'stylesheet alternate'
> combination isn't specific to how it's serialised, but is tied to
> the occurrence of the links. I.e., when both relations are present
> in links between the same resources, these special semantics take
> affect. However, this does seem to directly conflict with the HTML4
> language (see link above), so I don't think doing so is viable.
>
> Comments?
>
> --
> Mark Nottingham http://www.mnot.net/
>
>
--
Mark Nottingham http://www.mnot.net/